1. Field of the Invention
The present invention relates to determination of the refractive index of a medium and provides a method and a refractometer for absolute measurement of the refractive index.
2. Description of the Prior Art
Current methods of determining the refractive index of a medium are based on Descartes' laws describing the refraction phenomenon of passage of a light ray through an interface between the medium to be studied and a medium of known refractive index. These methods generally measure characteristics of the ray refracted by the interface to determine the refractive index of the medium being studied. More particularly, some methods exploit the existence of a limit angle of refraction. These methods determine the angle of an incident ray for which reflection is total and refraction non-existent. This angle corresponds to what is referred to as critical angle.
The refractometers measuring characteristics of a refracted ray or of the critical angle can be classified into two instrument categories.
On the one hand, there are refractometers performing absolute measurement of the refractive index. These tools allow, among other things, to control the quality of a product, to identify a component or to determine the proportions of the various components of a mixture. They are commonly used in the chemical, pharmaceutical and food-processing industry, and in analysis laboratories.
On the other hand, there are refractometers performing differential refractive index measurement between two media. The most widely known application of this category of instruments is analysis of the composition of a liquid solution after separation, by chromatography, of the different components upon passage of the liquid solution on an adsorbent solid.
EP-043,667 describes a refractometer using the comparison between the intensity of a first reflected ray and the intensity of a second reflected ray. The intensity of the first reflected ray depends on the refractive index of the medium being studied, whereas the intensity of the second reflected ray is independent of the refractive index of the medium studied. However, the intensity value of the second ray is totally independent of the intensity of the first ray. Intensity measurement of two different light rays notably has the drawback of following a complex procedure for measuring a refractive index and of losing measuring precision.
U.S. Pat. No. 3,650,631 describes a refractometer using the comparison between a first reflected ray and the intensity of a second ray reflected on an interface between the medium studied and a reference solid. The first ray falls onto the interface with an angle of incidence which is smaller than the critical angle, and the second ray falls onto the interface with an angle of incidence larger than the critical angle. The refractometer described in the '631 patent requires a critical angle. Now, the critical angle is determined by the refractive indices of the media that constitute the interface. This requirement therefore imposes limitations on the refractive index value measurable by the refractometer of the '631 patent. Furthermore, determination of the refractive index is based on the measurement of the intensity of the second ray which undergoes a total reflection at the interface. Now, in total reflection (angle of incidence larger than the critical angle), the evanescent wave appears, i.e. a phenomenon where the incident ray of the reference medium passes into the medium to be studied before it is reflected in the reference medium. In case of a highly absorbent studied medium, the intensity of the second reflected ray is attenuated and determination of the refractive index loses precision.
However, several limitations of use exist for refractometers measuring characteristics of the refracted light ray or the critical angle.
One limitation concerns the restricted measuring range of the refractive index of the medium studied. An absolute-measuring refractometer generally scans a range between approximately 1.3 and 1.7 RIU (Refractive Index Unit) for liquid media and between approximately 1.1 and 1.2 RIU for gaseous media, with a sensitivity of about 10−4 RIU. The measuring range of a differential-measuring refractometer extends approximately over 10−3 RIU with a sensitivity of 10−7 RIU.
Furthermore, measurement based on the characteristic of the refracted ray prevents determination of the refractive index of an opaque medium because the refracted ray is absorbed by the opaque medium. By determining the angle from which reflection is total, the evanescent wave (a phenomenon which causes passage of the incident ray at an interface from the first medium into the second medium before it is reflected in the first medium) is also absorbed by the second medium if it is opaque. Thus, measurement of the refractive index of media such as, for example, crude petroleum, inks and paints is not possible with such instruments.
Moreover, determination of the refractive index of a flowing fluid can be imprecise. In fact, if the fluid has no homogeneous optical characteristics as a result of its flow, the refracted ray or the evanescent wave propagated in the fluid undergoes alterations. The changes in the characteristics of the refracted ray or of the evanescent wave become greater when the flow of the fluid is turbulent.
For the same reasons as with determination of the refractive index of a flowing fluid, determination of the refractive index of a dispersed medium can be imprecise because dispersed media have no homogeneous optical characteristics.
The present invention provides a method and a refractometer allowing absolute measurement of a refractive index and overcoming the limitations of the prior art.